Question: Why they are different?
Answer: They are different because the Emotiv is modified into Easymotiv and Markus Kroesen was developing P300-based BCI speller.
Read below.

Question: Are there different effect in motor imagery using Emotiv system? Do experiment!
Answer: I am considering other hardware than Emotiv, i.e. mBrainTrain, because since August 2014, the prices are not so different.

See previous post about electrode position for mu wave
http://drhdrdro.wordpress.com/2014/08/19/eeg-electrode-placement-for-mu-wave-y01-m11-d19/
http://drhdrdro.blogspot.com/2014/08/eeg-electrode-placement-for-mu-wave-y01.html

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My other thought about electrode position for P300
http://drhdrdro.wordpress.com/2014/01/08/eeg-electrode-placement-for-p300-y01-m05-d08/
http://drhdrdro.blogspot.com/2014/01/eeg-electrode-placement-for-p300-y01.html
http://drhdrdro.wordpress.com/2013/12/18/todo-y01-m04-d18/
http://drhdrdro.blogspot.com/2013/12/todo-y01-m04-d18.html